A guide to the basic components of a BMX bicycle

The Frame:

BMX frames are made from chromoly steel or aluminum, with titanium, magnesium, and some plastic compounds used in different parts of the frame. High tensile steel is also used for frames in lower end bikes.The BMX frame is composed of two vertical tubes, the ‘seat tube’ in the center, and the ‘head tube’ at the front. The pedals attach through the ‘bottom bracket’ at the bottom of the seat tube. These vertical tubes are connected by six horizontal tubes, two between the head tube and the seat tube, called the ‘top tube’ and the ‘down tube’, and two on each side behind the seat tube that hold the rear tire, called ‘chain stay’ on the bottom and ‘seat stay’ at the top.

The Forks:

The forks go through the head tube vertically, contain the ‘steering tube’ and hold the front tire in place with 3/8” or 14mm ‘dropouts’, or slots for the axle. The handlebars are attached to the top of the steering tube. Handlebars come in one, two, or four pieces, with the two-piece handlebars most common. Holes in the head tube are for attaching the ‘gyro’, a device that keeps the cables from tangling.

The wheels consist of rims, spokes/nipples, tires, axles, and hubs. The rims are single, double, or triple walled with either 36 or 48 holes for spokes. The spokes are secured to the rim with the nipples. An axle attaches the front wheel to the frame, and a ‘hub’ is the drive mechanism in the rear wheel. Hubs provide the drive from the chain and come in different types, ‘cassette hubs’, ‘freewheel’ and ‘freecoaster’. Hub bearings come sealed or unsealed, with either a 3/8 inch or a 14mm axle. Tires are for either street or dirt; street tires are smooth and usually take 80-120 psi, while dirt tires are knobby and take 60psi.

The pedal mechanism is either American, Mid, Spanish, or European. The bearings in the bottom brackets come pressed or threaded into the frame. American and European bottom brackets use ‘cups’ to hold the bearings, while Mid and Spanish are pressed directly into the frame.

The Head set; the bearings in the head tube that rotate the steering tube, are either integrated or regular. Integrated bearings are molded into the frame; regular bearings are press fit into the frame.

The brakes used depend on the use of the bike. ‘U’ brakes are for street, dirt and flatland bikes, ‘V’ brakes are used for racing.